Patterns of sexual variation in hominoid mandibular morphology: a framework for interpreting the hominin fossil record

Abstract

For many species, sexual dimorphism is one of the major sources of intraspecific variation. This is the case in some extant great apes, such as gorillas and orangutans, and to a lesser degree in humans, chimpanzees and bonobos. This variation has been well documented in various aspects of these species skeletal anatomy, including differences in the size and shape of the body, cranium, canines, and cresting of males and females, but less is known about sexually dimorphic variation of great ape mandibles. This is particularly important for building robust analog models to interpreting variation in the early hominin fossil record which preserves a large proportion of isolated mandibles and partial mandibles. Here we describe the phenotypical expression of sexual dimorphism in the mandible of six extant hominoid species, including humans, using geometric morphometrics. Our analyses show that the extent of sexual dimorphism in mandibular size and shape amongst the species studied is not the same, as well as the presence of significant differences in the degree of sexual dimorphism being expressed at different sections of the mandible. Furthermore, we find significant differences in how sexual dimorphism is expressed phenotypically even amongst closely related species with small divergence times. We discuss the potential pathways leading to such variation and the implications for extinct hominin variability.